The commercial incentives for a process to convert synthesis gas to liquid fuels and other products are increasing as the need for energy sources increases. One successful approach to meeting this need has been to make synthesis gas and then synthetically convert the synthesis gas into heavier hydrocarbons (C5+) through the Fischer-Tropsch (F-T) process. The synthetic production of hydrocarbons by the catalytic reaction of synthesis gas is well known and is generally referred to as the Fischer-Tropsch reaction. This process was developed nearly eighty years ago in Germany, and since then, it has been practiced commercially in Germany during World War II and later in South Africa.
Fischer-Tropsch hydrocarbon conversion systems typically have a synthesis gas generator and a Fischer-Tropsch reactor unit. In the case of starting with a gas feed stock, the synthesis gas generator receives light, short-chain hydrocarbons such as methane and produces synthesis gas. The synthesis gas is then delivered to a Fischer-Tropsch reactor. In the F-T reactor, the synthesis gas is primarily converted to useful C5+ hydrocarbons. Recent examples of Fischer-Tropsch systems are included in U.S. Pat. Nos. 4,883,170; 4,973,453; 5,733,941; and 5,861,441, all of which are incorporated by reference herein for all purposes.
Numerous types of reactor systems have been used for carrying out the Fischer-Tropsch reaction. See generally the many examples found on www.fischertropsch.org. The commercial development of the Fischer-Tropsch reactor systems has included conventional fixed-bed and three-phase slurry bubble column designs or other moving-bed designs. But, due to the complicated interplay between heat and mass transfer and the relatively high cost of Fischer-Tropsch catalysts, no single reactor design has dominated the commercial developments to date.
Fischer-Tropsch three-phase bubble column reactors or the like appear to offer distinct advantages over the fixed-bed design in terms of heat transfer and diffusion characteristics. One particular type of three-phase bubble column is the slurry bubble column, wherein the catalyst size is generally between 10 and 200 microns (μM). Three-phase bubble column reactors present a number of technical challenges.
The technical challenges associated with three-phase bubble columns include solids management. One particular challenge in this area is to efficiently rejuvenate slurry catalysts. When a slurry Fischer-Tropsch catalyst is used over time, it has a disadvantage of slowly, but reversibly, deactivating compared to its initial catalytic activity. As the synthesis gas (primarily H2 and CO) is fed to the Fischer-Tropsch reactor and converted with the F-T catalyst, the catalyst experiences deactivation caused by carbon build up, physical degradation, and the effects of trace compounds other than CO and H2, such as by nitrogen containing species or oxygenated byproducts. “Carbon build up” references the accumulation of heavy hydrocarbons and carbonaceous type material that can have a hydrogen content less than that of F-T products. To remedy the deactivation, the catalyst is regenerated, or rejuvenated, using any of a number of techniques.
Rejuvenation is different from the initial activation of the Fischer-Tropsch catalyst. For cobalt catalysts, the initial activation involves converting the cobalt to a reduced state. An example of an initial activation technique is found U.S. Pat. No. 4,729,981, entitled “ROR-Activated Catalyst for Synthesis Gas Conversion,” which describes the initial preparation of a cobalt or nickel based Fischer-Tropsch catalyst by reducing it in hydrogen, oxidizing it in an oxygen-containing gas, and then reducing it in hydrogen. The catalyst is then ready for its initial use. Once in use, it will begin to deactivate, and it will need regeneration.
Regeneration of a Fischer-Tropsch catalyst after activation and operation has long been known to restore the activity of the catalyst. See, e.g., H. H. Storch et al., The Fischer-Tropsch And Related Synthesis (Wiley: New York 1951), 211–222. Storch describes using hydrogen treatments to restore the catalyst activity. There are many other examples. For example, U.S. Pat. No. 2,159,140 describes pulling the catalyst from the reactor (where it appears to have been fluidized) and removing the catalyst and treating it with hydrogen to regenerate the catalyst. U.S. Pat. No. 2,238,726 indicates that the non-volatile reaction products can be removed from the catalyst by treating it with hydrogen or gases or vapors containing hydrogen and that this can be done in the midst of oil circulation. Col. 2:34–54. As another example, U.S. Pat. No. 2,616,911 describes oxidizing the catalyst and then reducing it while maintaining it in suspension or a fluidized state. Other examples relating to regenerating and/or de-waxing Fischer-Tropsch catalysts include U.S. Pat. Nos. 6,323,248 B1; 6,201,030 B1; 5,844,005; 5,292,705; 2,247,087; 2,259,961; 2,289,731; 2,458,870; 2,518,337; and 2,440,109.
Regenerating a slurry catalyst presents particular challenges because the catalyst is in slurry form. Elaborate efforts have been made to separate the catalyst to allow regeneration outside the Fischer-Tropsch reactor or to regenerate it in-situ. The rejuvenation can be carried out intermittently or continuously.
As an example of a regeneration process, U.S. Pat. No. 5,973,012 describes a reversibly deactivated, particulate slurry catalyst that is rejuvenated by circulating the slurry from a slurry body through (i) a gas disengaging zone to remove gas bubbles from the slurry, (ii) a catalyst rejuvenation zone in which a catalyst rejuvenating gas contacts the catalyst in the slurry to rejuvenate it and to form a rejuvenated catalyst slurry, and (iii) a means for returning catalyst to the slurry body. This design appears to be primarily for use as in-situ regeneration design. The “in-situ” regeneration offers the advantage of keeping the catalyst in the slurry matrix; however, it presents many challenges. Amongst other challenges in-situ regeneration, the H2 partial pressure in the process is limited due to the low solubility of H2 in the liquid phase. Typically, the H2 partial pressure exposed to the catalyst within the liquid phase is less than about 10% of that in the gas phase. In addition, the hydrogen used to regenerate may modify the H2:CO ratio in the reactor for some time. Further still, the temperature may be limited by the boiling point and/or cracking properties of the liquid slurry constituents. For these reasons, “in situ” regeneration has real limitations.